According to a recent study a gallon of gas is also not a gallon. Gasoline is a liquid and expands and contracts with the heat. When the station buys gas it buys a gallon based on a set temperture. This means they alwasys get a gallon. However, when you pump you get a gallon based on the temperature at that given time. Best results are when it is cooler. So buy in the AM because even though it gets cooler in the evening the gas is still probably expanded. They did not measure whether you get increased performance with the gas when it is expanded or contracted or that might be the same thing